In the related art, a steam turbine which extracts rotation energy from pressure energy of steam is known. A turbine rotor in the steam turbine is exposed to steam having a high temperature, and thus, the turbine rotor requires an improved high-temperature resistance. For example, Patent Document 1 discloses low-alloy heat resistant steel having an improved high-temperature resistance as a material used for a turbine rotor.
In the related art, in a large-sized steam turbine (condensate turbine), a large-sized turbine rotor having an outer diameter dimension exceeding 1000 mm is used. In the large-sized turbine rotor, in order to improve a high-temperature resistance, a method such as so-called gradient quenching (gradient heat treatment) is used.
By performing the gradient quenching, it is possible to produce a turbine rotor having high-temperature resistances required for a high-pressure portion and a low-pressure portion of a turbine for each portion of one turbine rotor.